Liberal Democrats and environmental groups are urging the next Congress to do more to push back against the effects of climate change after watching the issue get sidelined over the first two years of the Trump administration.

Sen. Bernard Sanders of Vermont hosted a town hall Monday where he and other “climate justice” warriors, including Rep.-elect Alexandria Ocasio-Cortez of New York, warned that the longer President Trump and elected leaders refuse to address the “great crisis,” the more that people, the planet and the economy will be hurt.

“The time is late,” Mr. Sanders said. “Countries all over the world are going to have to stand up [and] take on the fossil fuel industry if we are going to leave our kids and our grandchildren a planet that is habitable.

“This is a crisis situation,” he said. “It is unprecedented and we have got to act in an unprecedented way.”

The town hall event coincided with the start of the United Nations climate change summit in Poland and came about a month after Democrats took control of the House leaving activists excited about the role that Ms. Ocasio-Cortez and other activists turned politicians could play in focusing more attention on the issue.

Ms. Ocasio-Cortez highlighted her push for a “Green New Deal” that calls for a select committee to put together a plan to combat climate change by 2020.

“This is going to be the great society, the moon shot, the civil rights movement of our generation,” she said.

The calls for action on climate change spiked last month after 13 federal agencies released a report the Fourth National Climate Change Assessment that provided stark warnings about how rising temperatures could lead to more drought, fires and flooding, and devastate the economy and people’s health.

Mr. Trump, who has rolled back regulation targeting carbon emissions, dismissed the findings and GOP lawmakers have basically followed suit.

Likely contenders for the Democratic presidential nomination have pounced on the report, and the issue. Former New York City Mayor Michael Bloomberg, is slated on Tuesday to be in Iowa to screen a new documentary called “Paris to Pittsburgh” that highlights the impact of climate change and what people are doing to address the problem.

The title of the documentary, which Mr. Bloomberg helped finance, alludes to Mr. Trump’s announcement last year that he was pulling the U.S. from the Paris climate agreement because, “I was elected to represent the citizens of Pittsburgh, not Paris.”

“Iowans understand what too many leaders in Washington don’t: Fighting climate change is good for our health and our economy,” Mr. Bloomberg said Monday in a Des Moines Register op-ed.

Sen. Elizabeth Warren of Massachusetts, another likely 2020 contender introduced legislation this year that would mandate public companies disclose more information about their exposure to climate-related risks. Half the bill’s eight co-sponsors Sens. Cory Booker of New Jersey, Jeff Merkley of Oregon, Kamala D. Harris of California and Kirsten Gillibrand of New York are thought to be lining up presidential bids.

Democratic leader Nancy Pelosi, meanwhile, has called on her caucus to revive a select committee on climate change that was defunded when Republicans took over the chamber, and others have vowed to tackle the issue in congressional hearings.

It remains to be seen how much could change legislatively in Washington next year particularly with Republicans calling the shots in the Senate.

What is clear is that climate is poised to get more of a look in the Democrat-controlled House, where lawmakers already are laying out plans to hold a series of congressional hearings on the subject early next year.

Ms. Ocasio-Cortez helped thrust the issue into the headlines last month when she joined grass-roots activists protesting outside Mrs. Pelosi’s office last month demanding more action on climate change.

“I believe that the progressive movement is the only movement that has answers right now,” Ms. Ocasio-Cortez said Monday.